Smallthoughts has this nice musing about the role of email as universal life interface:
I find myself using email as a quasi-database all the time: to make lists, send myself reminders, record important snippets of information, even log hours spent on a project. Why? First of all, because the email client is the one window that I will always have open and easily accessible, whatever else I might be doing. Second, email is a great redundant data source: as well as going to whoever (or whatever) I send it to, a copy of the message ends up on my local machine, another on my servers, maybe yet another on Google’s servers, and so on. If I need to, chances are I can find it later from pretty much anywhere. But probably most importantly, email is, for many of us, the main personal input stream: if some new piece of information comes up that I have to record or take action on, chances are pretty good it came to me through email. So what more natural way to respond than right from the email client?